1. Field
The disclosed concept relates generally to electrical switching apparatus and, more particularly, to circuit interrupters. The disclosed concept further relates to circuit interrupters providing grounded neutral protection. The disclosed concept also pertains to methods of controlling circuit interrupters providing grounded neutral protection.
2. Background Information
One type of electrical switching apparatus is a circuit interrupter. Circuit interrupters, such as for example and without limitation, circuit breakers, are typically used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Circuit breakers typically include separable contacts. The separable contacts may be operated either manually by way of a handle disposed on the outside of a case or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, such as a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
One type of fault condition is a grounded neutral condition. UL943 specifies that a circuit interrupter should trip when the impedance between neutral and ground downstream of the circuit interrupter is 2 ohms or less. UL943 also specifies that the circuit interrupter must be capable of causing this trip without the application of a load current. However, without the application of a load current, the ground fault current is relatively small, thus making it difficult to detect a grounded neutral condition.
FIG. 1 is a circuit diagram of a system including a prior circuit interrupter 1 which is capable of detecting a grounded neutral condition without the application of a load current. The circuit interrupter 1 is electrically connected to a power source 2, a neutral 3, and a ground 20 on its upstream portion. The impedance between the load neutral and ground 20 on the downstream portion of the circuit interrupter 1 is represented by R1. The circuit interrupter 1 includes first and second electrical conductors 5,6 electrically connected to outputs of the power source 2. The circuit interrupter 1 also includes separable contacts 7 and an operating mechanism 8 which is configured to open and close the separable contacts 7. The circuit interrupter 1 further includes a trip circuit which is electrically connected between the first and second electrical conductors 5,6. The trip circuit includes a trip actuator 9 which cooperates with the operating mechanism 8 to trip open the separable contacts 7. The trip actuator 9 includes a trip coil 10 which initiates tripping of the separable contacts 7 when sufficient current is passed therethrough. The trip actuator 9 also includes a silicon controlled rectifier (SCR) 11 which turns on and off to control whether current passes through the trip coil 10.
The circuit interrupter 1 includes a ground fault detection circuit which is capable of detecting a grounded neutral condition. The ground fault detection circuit includes a ground fault sensor 12 which senses a ground fault current as a difference between the current passing through the first electrical conductor 5 and the second electrical conductor 6. The ground fault detection circuit also includes an amplifier circuit 13 that converts the sensed ground fault current to a voltage and outputs it to a comparator circuit 16.
The ground fault detection circuit further includes a capacitor 14 and a grounded neutral transformer 15 which are electrically connected in series combination to the output of the ground fault amplifier circuit 13. The grounded neutral transformer 15 causes a positive feedback condition for the ground fault amplifier circuit 13, thus resulting in an unstable condition. More particularly, the ground fault amplifier circuit 13 oscillates at about the resonant frequency of the series combination of the capacitor 14 and grounded neutral transformer 15. The unstable condition causes an increase in the output of the ground fault amplifier circuit 13.
The comparator circuit 16 compares the output of the ground fault amplifier circuit 13 to a threshold voltage. When the output of the ground fault amplifier circuit 13 exceeds the threshold voltage, the comparator circuit 16 outputs a signal to the gate of the SCR 11 to turn on the SCR 11, thus allowing current to pass through the trip coil 10 and cause the separable contacts 7 to trip open.
Based on the increase in the output of the ground fault amplifier circuit 13 due to the instability of the ground fault detection circuit, the comparator circuit 16 is able to turn on the SCR 11 even when the ground fault current is relatively small. However, the addition of the capacitor 14 and grounded neutral transformer 15 to cause the instability results in an increase in the cost of the circuit interrupter 1.
There is room for improvement in circuit interrupters.